Light scattering and ultra-violet absorption studies on dilute aqueous solutions of poly-4-vinylpyridinium chloride

Light Scattering and Ultra-violet Absorption Studies on Dilute Aqueous Solutions of Poly-4-vinylpyridinium Chloride D . O . JORDAN, T . KURUCSEV a n d

R . L . DARSKUS*

The discontinuous concentration of viscosity, streaming birefringence and conductance of dilute aqueous solutions of poly-4-vinylpyridinium chloride is attributed, on the basis of light scattering and spectra measurements, to the extensive hydrolysis of the poly-ions in dilute solution. The decrease in the charge on the poly-ion consequent on the hydrolysis produces changes in the shape and size which may explain the observed hydrodynamic properties.

THE discontinuous concentration dependence of the viscosity, streaming birefringence and conductance of dilute aqueous solutions of poly-4-vinylpyridinium chloride has recently been described 1' 2. The presence of similar discontinuities is also suggested by the published reduced viscosity data at low concentrations for many strong and weak polyelectrolytes; the effects observed for poly-4-vinylpyridinium chloride solutions have therefore been assumed to represent a general property of polyelectrolyte solutions. By analogy with the behaviour of micelle-forming soaps and detergents, the discontinuities were explained tentatively in terms of the reversible aggregation of the poly-ions in solution. For such aggregation to occur, however, it is necessary to assume the presence of attractive forces between poly-ion segments which is contrary to accepted theories of polyelectrolyte solutions. We have now determined the size of the polyvinylpyridinium ions in solution and find no direct evidence for aggregation. The spectra of dilute solutions of poly-4-vinylpyridinium chloride suggest that extensive hydrolysis of the polyelectrolyte ions occurs. This has been confirmed by electrochemical measurements. The previously published hydrodynamic properties have therefore been re-interpreted in terms of the varying degree of hydrolysis of the polyvinylpyridinium ions. EXPERIMENTAL

Several preparations of poly-4-vinylpyridine (PVP) were made by emulsion polymerization ~ of freshly distilled 4-vinylpyridine (Light and Co.) with either benzoyl peroxide or azobisisobutyronitrile as initiator. The polymer samples were fractionated by precipitation from methanol solution with toluene, the fractions redissolved in tertiary butanol and freeze dried. The purity of the samples was determined by potentiometric titration 4 with p-toluene sulphonic acid in glacial acetic acid solution using a silver/ silver chloride reference electrode and a glass indicator electrode. In this system the products of the titration do not precipitate. Satisfactory titration end points (+_ 1 per cen0 compared with the potentiometric equivalence points were also obtained visually using crystal violet as indicator. The *Present address : Institut fiir tahysikalische Chemie der Universitlit, Maim, Western Germany.

303

D. O. JORDAN, T. KURUCSEV and R. L. DARSKUS impurity present in the samples (8 to 12 per cent) was assumed to be water, an assumption consistent with the results of elementary analysis and with the slightly hygroscopic nature of the polymer. The fractions were characterized by their intrinsic viscosities in ethanol at 25°C determined with Ostwald capillary viscometers. The solution viscosities of the high molecular weight fractions ([7] > 1 000 ml/g) were found to decrease with time. This instability of solutions of PVP has been observed previously 3,5 and explained in terms of degradation due to the presence of peroxides in the polymer chains. Oxidative degradation is probably an additional factor contributing to this instability since the effect was found to diminish by keeping the solutions under nitrogen. None of the polymer fractions used in the study discussed below showed any detectable variation of the viscosity with time. Poly-4-vinylpyridinium chloride (PVP-HC1) was prepared by the partial neutralization of PVP with dilute hydrochloric acid solution. The dissolution of the samples generally required several days. Concentrations (c) for both PVP and PVP-HC1 are expressed as grammes of uncharged polymer per miUilitre of solution. Light scattering measurements were made with a modified P.C.L.-Peaker light scattering apparatus ~,7. A narrow spectrometer slit and four additional apertures were introduced into the optical system to ensure a more precise definition of the scattering volume and a reduction in stray light. The effect of fluctuations in the intensity of the light source was reduced by directly measuring the ratio of the photocurrents due to the incident and scattered rays. This photocurrent ratio measuring device (manufactured by EILCO, South Australia) was tuned to receive only the 100c/s component of the photocurrent. Since the mercury arc source (Mazda 250 W type M E / D ) was run from a 50 c/s a.c. supply, the d.c. component of the photomultiplier tube dark current was eliminated. Scattering cells, which were of rectangular shape and had a volume of 4 ml, were constructed from 3-5 mm thick glass. The cells were held in an outer cell of blackened brass which contained distilled water while measurements were being made. Unpolarized light of 436 m~ was used. After traversing the scattering cell, the primary beam was absorbed by red glass. With this arrangement the Fresnel correction 8 was rendered negligible. The scattered light was observed through a semicircular glass window cemented to the outer cell. The range of angles through which observation was possible was from 27 ° to 140 ° . The dependence of the scattering volume on the angle of observation, tg, was determined empirically using dilute solutions of sodium dichlorofluoresceinate. The deviations from proportionality to 1/sin ,0 were less than three per cent. The instrument was calibrated with benzene using the mean value of 47x 10 ~ cm -1 for the reduced scattering intensityS.% The refractive index correction was taken to be nL Clarification of the PVP-HC1 solutions was carried out using a Spinco model E ultracentrifuge with an SW-39 swinging bucket rotor at 100 000 g for two hours. In preliminary experiments, when these solutions were clarified in fixed angle rotors at 20000g, 'striations' were sometimes observed similar to those reported previously 1°. 304

D I L U T E A Q U E O U S SOLUTIONS O F POLY-4-VINYLPYRIDINIUM C H L O R I D E

Specific refractive increments were determined with a double prism differential refractometer of the type described by Cecil and Ogston 1'. Spectrophotometric measurements were made using a Unicam SP 500 instrument with constant temperature attachment. The photoelectric response of the instrument was tested in the wavelength range 215 to 500 ml~ by standard potassium chromate solution 1:. Some of the measurements were repeated on an Optica recording spectrophotometer. The extinction coefficients, defined by K=(1/l)log(lo/1), where 1 is the optical path length through the solution, were found to be independent of time. Conductance measurements were made using a cell designed to avoid errors due to adsorption of the solute on the electrodes. The concentration was changed by adding polyelectrolyte solution or solvent to the cell in an atmosphere of purified nitrogen. The bridge employed has been described previouslyl'L Measurement of pH was made with a Cambridge pH-meter using an 'alkacid' glass electrode. I

RESULTS

AND

.

DISCUSSION

The curve relating the relative viscosity with concentration of aqueous solutions of PVP at various degrees of neutralization cuts the concentration axis at a finite value of the concentration, 70, below which the viscosity of the solution is indistinguishable from that of the solvent ~'2. A similar effect is observed for the magnitude of the birefringence of the solutions and, further, the curve relating the specific conductance with concentration exhibits a sharp change of slope at the same critical concentration, 7o ~,'-' This discontinuous variation of solution properties with concentration must be related to changes in either the size or shape (or both) of the poly-ion kinetic unit in solution. An interpretation of these results was given 1,~ on the basis of the first of these alternatives, thus assuming that aggregation of the poly-ions occurred at concentrations greater than 3'0, such aggregation being enhanced by the presence of added electrolyte (i.e. 70 is displaced to lower concentrations on the addition of electrolyte). A direct test of this interpretation is possible by determining the molecular weight of the polymer in solution under the following conditions: (i) in the uncharged state in a solvent where no aggregation would occur, (ii) in the partially neutralized and therefore charged state at concentrations above 70, and (iii) at the same degree of neutralization but at a concentration below "r0- Light scattering measurements have therefore been made on solutions satisfying these conditions. The weight average molecular weights were equated to the reciprocal of the ratio Kc/R 40) at zero angle and zero polymer concentration 8 and extrapolations were effected by Zimm plots 14. No corrections for depolarization were applied. The degree of polymerization of the PVP sample in ethanol obtained from light scattering measurements on four solutions in the concentration range 0.6 to 2.8x 10-4g/ml was 21000. Viscosity measurements on the same sample of PVP in ethanol solution gave a value, calculated by the method of Berkowitz, Yamin and Fuoss 3, of 16 000 which is regarded as being in reasonable agreement with the light scattering result. The degree of polymerization of the sample of PVP 56 per cent neutralized with hydrochloric acid in aqueous 0-2 M sodium chloride calculated from 305

D. O. J O R D A N , T. K U R U C S E V and R. L. D A R S K U S

light scattering measurements on five solutions in the concentration range 2 to 10 x 10 -4 g/ml was 16 000. This range of concentration lies well above the critical concentration for this solvent. The lower value obtained for the degree of polymerization of the polyelectrolyte in sodium chloride solution, compared with that obtained for the uncharged polymer in ethanol from light scattering measurements, is almost certainly due to the neglect of preferential interactions in the three-component systemlL However, even though the values of the degree of polymerization obtained lack precision owing to the nature of the solutions and to the low concentration range studied, these measurements are sufficient to indicate that no aggregation of the poly-ions has occurred in the presence of sodium chloride compared with the corresponding uncharged molecules in ethanol. Light scattering measurements have also been made on solutions of 50 per cent neutralized PVP at concentrations 0"18 and 0-29 × 10 -~ g/ml in pure water as solvent. These concentrations lie well below % for PVP for this degree of neutralization in pure water. Light scattering measurements at such low concentrations are subject to considerable error owing to the low intensity of the scattered light and the consequential large corrections for solvent scattering; the extrapolation to zero angle and zero polymer concentration therefore proved to be somewhat arbitrary. However, the value obtained for the degree of polymerization, 30 000, even though approximate is in accordance with the conclusion reached above that there is no aggregation of the poly-ion on increasing the concentration through the critical concentration, %. An alternative explanation of the discontinuous concentration dependence of solution properties was suggested from spectral studies. The ultra-violet spectra of aqueous PVP-HC1 solutions were found to be concentration dependent, as is shown in Figure 1, which gives the spectra of solutions of a

~-Gooo ._A C OJ U

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C 0

e I

~6 C

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2 0

220

240

260

280

Wavelength, m~

Figure / - - A b s o r p t i o n spectra of aqueous PVP-80 per cent hydrochloric acid at different concentrations: a 0'77, b 1"44, c 1"88, d 3-50, e 4"23× 10 -~ g / m l

306

DILUTE AQUEOUS SOLUTIONS OF POLY-4-V1NYLPYRIDINIUM CHLORIDE an 80 per cent neutralized PVP fraction in pure water. The molecular extinction coefficients are expressed in terms of the number of gramme atoms of nitrogen per litre of polymer solution. In Figure 2 the variation of the extinction coefficient for these solutions is shown for two different wavelengths and the discontinuous nature of the deviations from Beer's law is revealed. The concentration at which the change in slope occurs at these wavelengths has been studied as a function of temperature, degree of neutralization and molecular weight of the polymer and of the concentration of sodium chloride. This study has shown that the position of the point of discontinuity in the curves relating extinction coefficient and concentration may be identified with the critical concentration, 7,, previously identified by hydrcdynamic methods.

m

08

"~ 06

0

Figure 2--Variation of optical density oc- 04 with concentration of PVP-80 per .9 cent hydrochloric acid at two differ- "5 ent wavelengths: © 255 mF, • 225 m/z C Ld

02

2 Concentration, 105x c ( g / m [ )

4

The ultra-violet absorption spectra of solutions of PVP in ethanol and in 0" 1 M hydrochloric acid are shown in Figure 3. These spectra are considered to represent the extremes of uncharged polymer and fully ionized polyelectrolyte respectively. The molecular extinction coefficients were independent of concentration under these conditions. Comparison of the spectra shown in Figures l and 3 suggests that the concentration dependence of the spectral activity of PVP-HC1 solutions could be attributed to the variation of the degree of hydrolysis of the pyridinium groups. If it be assumed that the molecular extinction at some convenient wavelength, e.g. 255 m e or 225 m , , is a linear function of the fraction of ionized pyridinium groups, the degree of hydrolysis of the poly-ions m a y be calculated as a function of concentration. From the results obtained (Figure 4) it is evident that PVP-HC1 in pure water as solvent is extensively hydrolysed with only 15 to 20 per cent of the pyridinium groups remaining in the charged form in very dilute solution. Similar high values for the degree of hydrolysis have been obtained from conductance measurements. If the poly-ion at high dilutions is hydrolysed to a large extent, the contribution of the poly-ion to the conductance may be neglected and if the 3O7

D. O. J O R D A N , T. K U R U C S E V and R. L. D A R S K U S T 6

E u

000

C

.-~ ~ 000 ._

O

0 u tO

~ 2000 u

200

220

240 Wavelength, m~t

260

280

Figure 3--Absorption spectra of P V P in different solvents: a 0.1 M hydrochloric acid, b ethanol

effect of counterion binding is also, neglected the conductance of the solution may be attributed, to a first approximation, to that of the hydrogen and chloride ions only. The errors introduced by the two assumptions are to some extent mutually compensatory. The values of the degree of hydrolysis calculated in this way (Figure 4) again indicate that 80 to 85 per cent of the pyridinium groups are hydrolysed. Further evidence for the extensive hydrolysis of PVP-HC1 in aqueous solution has been obtained from pH measurements. On the basis of the above results it would appear that the previously described discontinuous dependence of hydrodynamic properties on concentration 1,2 is caused by changes in the shape of the kinetic unit in solution 100

tB

x-x-

-.

x---x-

x~

\ %

"13

8O

Q

60

2 4 Concentration, 105 × e(g/mt)

Figure 4--Variation of the degree of hydrolysis of P V P - 8 0 per cent hydrochloric acid with concentration: X, determined from conductance measurements (sample E6); determined from molecular extinction coefficients, ©, at 225 my, • at 255 m~ (sample R1)

308

DILUTE AQUEOUS SOLUTIONS OF POLY-4-VINYLPYRIDINIUM CHLORIDE due to changes in the degree of hydrolysis ~ and hence in the charge of the poly-ion as the concentration is altered. Owing to the experimental difficulties of observing the properties of very dilute solutions, the nature of the kinetic unit at concentrations below 3'0 has not been studied in detail. However, the intrinsic viscosity in ethanol of one fraction of PVP is 410 ml/g, while the same material in its polyelectrolyte form in pure water at a concentration below 3,o has a reduced viscosity of less than 40 ml/g (extrapolation to give the intrinsic viscosity is not possible for the polyelectrolyte in pure water as solvent). A similar marked reduction of the intrinsic viscosity has been observed for styrene-maleic acid copolymers '~ in transferring from a ,0 solvent to an acid aqueous solution. Accordingly, the polyelectrolyte at concentrations below 70 must be regarded as a tightly coiled structure. Since water is a non-solvent for uncharged PVP and since below the point of discontinuity only about 15 per cent of the original electrolyte groups will carry charges, this contraction of the polymer to well below the size that corresponds to a random chain configuration may be attributed to the hydrophobic bonding mechanism discussed by Kauzmann '~ for proteins. The polyelectrolyte unit at very low charge may thus be regarded as being an essentially spherical structure from the interior of which water is excluded. Coagulation of this system would be prevented by the presence of the charges on the particles. The apparent constancy of the degree of hydrolysis below 70 (Figure 4) could be explained if the remaining charged groups were buried inside the compact structure, either as such or intramolecularly hydrogen bonded, and thus inaccessible to the solvent. The hydrodynamic behaviour previously described' '~ can be satisfactorily interpreted on the basis of the change in shape accompanying dilution. Thus the maxima observed in the curve relating reduced viscosity with concentration for solutions of PVP-HC1 in pure water may be explained in the following way. With increasing dilution there will be two opposing effects. The poly-ions will at first expand on dilution due to the decrease in the screening effect of the counterions on the intramolecular electrostatic repulsion but as dilution proceeds, hydrolysis will increase with a consequential decrease in the number of charged groups, and hence a decrease in the intramolecular repulsion leading finally to a contraction of the poly-ion. At very low concentrations (corresponding to -/,,) where hydrolysis is high, the polyelectrolyte will collapse to a compact sphere such that the viscosity contribution will approach the Einstein limit. The increase of ~/,~ for PVP-HC1 with temperature I and the decrease on the addition of hydrochloric acid and sodium chloride 2 is also in agreement with the hydrolysis mechanism.

We wish t~ acknowledge the assistance of Mr B. C. Simpson who carried out many o[ the spectral measurements, and the award o[ a Commonwealth Postgraduate Scholarship to R.L.D. Department of Physical and Inorganic Chemistry, The Johnson Laboratories, University of Adelaide, Adelaide, South Australia. (Received October 1964) 309

D. O. J O R D A N , T. K U R U C S E V and R. L. D A R S K U S REFERENCES 1 JORDAN, D. O. and KURUCSEV, T. Polymer, Lond. 1960, 1, 193

JORDAN,D. O. and KURUCSEV,T. Polymer, Lond. 1960, 1, 202 a BERKOWrrz, J. B., YAMIN, M. and Fuoss, R. M. I. Polym. Sci. 1958, 28, 69 4 BURLEIGFI,J. E., McKINNEY, O. F. and BARKER, M. G. Analyt. Chem. 1959, 31, 1684 5 FIaT_~ERALD,L. B. and Fuoss, R. M. J. Polym. Sci. 1'954, 14, 329 BOSWORTH, P., MASSON, C. R., MELWLLE, H. W. and PEAKER, F. W. J. Polym. Sci. 1952, 9, 565 r OVERALL,D. W. and PEAKER, F. W. Makromol. Chem. 1959, 33, 222 s See, for example, STACEY, K. A. Light Scattering in Physical Chemistry. Butterworths: London, 1956 9 CONMON, D. J. J. Colloid Sci. 1960, 15, 408 lo See, for example, HUQUE, M. M., JAwoRzY, J. and GORING, D. A. J. Polym. Sci. 1959, 39, 9 11 CECrL, R. and OGSTON, A. G. J. sci. Instrum. 1951, 28, 253 12 HAUPT, G. W. J. opt. Soc. Amer. 1952, 42,441 la INMAN, R. B. and JORDAN,D. O. Biochim. biophys. Acta, 1960, 42, 421 14 ZIMM, B. n . J. chem. Phys. 1948, 16, 1093 and 1099 15 VRIJ, A. and OVERBEEK, J. TH. G. J. Colloid Sci. 1962, 17, 570 16 ALFREY, T. and MORAWETZ,H. J. Amer. chem. Soc. 1952, 74, 436 lr DANNHAUSER,W., GLAZE, W. H., DUELTGEN, R. L. and NINOMIYA, K. J. phys. Chem. 1960, 64, 954 18 KAUZMANN,W. Advances in Protein Chemistry, 1956, 14, 1

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